In devices of the character described, material to be mixed is placed in a mixing vessel and subjected to the rotary orbital movement of a mixing screw. Because of the load of the material in the vessel, as well as the characters of the material itself, the mixing screw is, on occasion, subjected to an excessive load which is demostrated by the generation of heat in its bearing mounting. Should this heat generation be ignored, damage to the bearing could occur with a resultant breakdown of the equipment as well as expensive loss of time and productivity.